In a conventional gas blast circuit breaker, an arc-extinguishing gas is simultaneously compressed upon an opening motion of contacts, and the compressed gas is directed by a throat portion of an insulated nozzle toward an arc generated by the electric current breaking operation between the separated contacts. A flow speed of the compressed gas is accelerated at the throat portion so that the flow speed thereof at a downstream side of the throat portion becomes subsonic or supersonic. When the throat portion is moving toward the outside of a stationary contact on which the high flow speed gas passes, a gas pressure at a part of a forward end of the stationary contact becomes transiently less than a filling gas pressure in the breaker, although a strength of electric field at the forward end of the stationary contact is high. Therefore, an electrical, insulating resistance between the contacts is decreased largely, and a small capacitive current interrupting characteristic, which is important when a distance between the contacts is small and the electric field strength between the contacts is large after the electric current is broken, particularly just after the throat portion has moved to the outside of the stationary contact, is deteriorated.
In a conventional gas blast circuit breaker as disclosed in Japanese Patent Unexamined Publication No. 60-150521, the gas blast circuit breaker has a reflection surface at a gas flow downstream side of a throat portion of an insulated nozzle to reflect the gas flow toward a stationary contact after the gas passes through the throat portion, so that a decrease in pressure at a forward end of the stationary contact is prevented and an electrical insulating resistance between the contacts is improved.
In a conventional gas blast circuit breaker as disclosed in Japanese Patent Unexamined Publication No. 61-206126, the gas blast circuit breaker has a bypass hole connecting an upstream side of a throat portion of an insulated nozzle to a downstream side thereof so that a high temperature gas in the upstream side of the throat portion is discharged through the bypass hole to accelerate a recovery of an insulating resistance between the contacts and the gas is directed toward the stationary contact to improve the insulating resistance between the contacts.